Ink jet printing is a non-impact printing process in which a digital signal produces droplets of ink on media substrates such as paper or transparency films. In thermal ink jet printing, resistive heating is used to vaporize the ink, which is expelled through an orifice in the ink jet printhead toward the substrate. This process is known as firing, in which water is vaporized by the heat, causing a very rapid and drastic local compositional change and temperature rise. This occurs repeatedly at high frequency during the life of the printhead. In the orifice areas, the ink composition can drastically change from water-rich to solvent-rich as well due to evaporation of water. This may cause the pigment particles to flocculate around the orifice in the printhead, eventually leading to plugging of the orifice in the printer mechanism from which droplets of ink are expelled. This will lead to misdirection of the ink drop or prevention of drop ejection.
In the conventional coating applications, many additives of organic nature are used to impart the desired physical properties for the final use. Examples include polymer binders, thickeners, thixotropic agents, coating aids, etc. During the drying process, these components are concentrated. The pigment dispersion needs to accommodate such changes in order to maintain the uniformity and color quality for the final coatings.
Aqueous pigment dispersions are well known and have been used commercially in films such as paints on substrates. A pigment dispersion is generally stabilized by either a non-ionic or ionic technique. In the non-ionic technique, the pigment particles are stabilized by a polymer that has a water-soluble, hydrophilic section that extends into the water and provides entropic or steric stabilization. Representative polymers useful for this purpose include polyvinyl alcohol, cellulosics, and ethylene oxide modified phenols. While the non-ionic technique is not sensitive to pH changes or ionic contamination, it has a major disadvantage for many applications in that the final product is water sensitive. Thus, if used in ink applications or the like, the pigment will tend to smear upon exposure to moisture.
In the ionic technique, the pigment particles are stabilized by a polymer of an ion containing monomer, such as neutralized acrylic, maleic, or vinyl sulfonic acid. The polymer provides stabilization through a charged double layer mechanism whereby ionic repulsion hinders the particles from flocculation. Since the neutralizing component tends to evaporate after application, the polymer then has reduced water solubility and the final product is not water sensitive. Ideally, a polymer dispersant which could provide both steric and charged double layer stabilization forces would make a much more robust pigment dispersion.
Polymer dispersants having both random and block structures have been disclosed. For example, U.S. Pat. No. 4,597,794 proposes an aqueous ink dispersion for ink jet printers in which the pigment is contained in a polymer having ionic hydrophilic segments and aromatic hydrophobic segments that adhere to the pigment surface. U.S. Pat. No. 5,085,698, the disclosure of which is incorporated herein by reference, teaches the use of AB or BAB block polymers as pigment dispersants for aqueous ink jet inks. While both the random and block polymer dispersants offer improved stability for the dispersed pigment, further improvements are desired for more advanced high quality coating applications.
Accordingly, the need exists for dispersants which will provide improved dispersion stability, flexibility for adjusting to a changing environment and which provide dispersions with improved coating quality.